The present invention relates generally to protective methods and circuits for sheathed electrical resistance heating units. More particularly, the invention relates to a method and circuits for reliably interrupting a fault in the form of an arcing short which may occur between the heating element and the grounded conductive outer sheath of such a heating unit, regardless of where along the length of the heating unit the fault occurs. The protective methods and circuits of the invention are particularly beneficial when operated from a 120 volt unbalanced power line. The invention may be employed in household appliances, particularly dishwashers.
Sheathed electrical resistance heating units have been employed for many years in various products, for example electric ranges and dishwashers. Such heating units generally comprise a heating element in the form of a spiralled electric resistance wire encased in an elongated ceramic-filled metal outer sheath, which is electrically conductive. The ceramic material transmits heat, but is an electrical insulator. Thus, the outer sheath becomes hot, but normally remains electrically insulated from the heating element. A suitable ceramic material is magnesium oxide. Sheathed electrical resistance heating units of this general type are described in U.S. Pat. No. 2,094,480 to Vogel and U.S. Pat. No. 3,592,771 to Vedder et al, the disclosures of which are hereby incorporated by reference.
In the operation of such heating units, the terminals of the heating element are connected to a power source, for example a household AC power line. A 120 volt heating unit is normally connected between the "hot" side and neutral of a 120 volt unbalanced-to-ground line, in which case one terminal of the heating element is electrically "hot," and the other terminal is electrically "cold." A 240 volt heating unit is normally connected across both sides of a 240 volt balanced line, in which case both terminals of the heating element are "hot." Normally, for safety reasons, the outer conductive sheath is grounded.
One particular failure mode which is possible in such a heating unit is associated with a breakdown in the insulation qualities of the magnesium oxide separating the heating element from the outer sheath, accompanied by arcing between the element and the outer sheath. For reasons not fully understood, such a breakdown and arc may occur even near the "cold" end of the heating unit where, theoretically, there should be little potential difference between the heating element and the outer sheath. When such a fault occurs near the heating unit "cold" end, current drawn from the power line is not significantly increased because a substantial portion of the resistance heating element remains interposed in series with the "hot" side of the power line. Any circuit breaker or fuse device in series with the "hot" side of the power line is not immediately tripped or "blown," and the arc is thus not immediately extinguished.
In some cases, the arc generates sufficient heat to actually melt the outer sheath. Such disruption in the outer sheath travels lengthwise along the sheath, towards the "hot" end, until only a short length of heating element remains in the circuit. The resulting low resistance allows sufficient current to flow to cause the protective fuse or circuit breaker to interrupt the circuit. This phenomenon is commonly referred to as "zippering" because the disruption in the outer sheath resembles the opening of a zipper.
In the event a fault occurs near the "hot" end of the heating unit, the fuse or circuit breaker interrupts the circuit in fairly short order, and significant "zippering" does not occur.
One device to rapidly terminate "zippering" is disclosed in U.S. Pat. No. 4,044,224, issued to Jenkins and Herbst. The Jenkins and Herbst device replaces the direct electrical connection between the outer conductive sheath and ground with a fusible link. During normal operation of the heating unit when the insulation material is intact, substantially no current flows through the outer sheath ground connection (with the exception of a small amount of AC leakage current largely as a result of capacitive effects). However, when a fault occurs, significant current flows through the outer sheath ground connection, causing the fusible link to open. Additionally, in the Jenkins and Herbst arrangement, a switch is mechanically arranged to open when the fusible link is broken, and this switch cuts off power to the heating element. The particular application of the protective device described by Jenkins and Herbst is in a household dishwasher having a plastic tub. The present invention is an alternative to the device disclosed in the Jenkins and Herbst patent.
Accordingly, it is an object of the invention to rapidly terminate a fault between the heating element and the outer sheath of a sheathed electrical resistance heating unit when the fault occurs near either end of the heating unit.
It is another object of the invention to rapidly terminate a fault between the heating element and the outer sheath of a sheathed electrical resistance heating unit, regardless of where along the heating unit the fault occurs.
It is another object of the invention to provide low-cost circuitry for reliably and effectively extinguishing an arcing short within a sheathed electrical resistance heating unit regardless of where along the heating unit the arching short occurs.
It is still another object of the invention to provide such circuitry which is suitable for use with a sheathed electrical resistance heating unit which is operated alternately by connecting it directly across a power source for full-wave, full power operation, or by connecting the heating unit in a half-wave rectifying arrangement for partial power operation.